Europa and Io are somewhat
similar in bulk composition to the
terrestrial planets: primarily composed of
silicate rock.
Unlike Io, however, Europa has thin outer layer of ice.
Recent data from Galileo indicate that
Europa has a layered internal structure perhaps with a small metallic core.

But Europa's surface is not at all like anything in the inner solar system.
It is exceedingly smooth:
few features more than a few hundred meters high
have been seen. The prominent markings seem to be only
albedo features or very low relief.

There are very few craters on Europa; only three
craters larger than 5 km in diameter have been found.
This would seem to indicate a young and
active surface.
However, the Voyagers mapped only a
fraction of the surface at high resolution.
The precise age of Europa's surface is an open question.

The images of Europa's surface strongly resemble images of sea ice on Earth.
It is possible that beneath Europa's surface ice there is a layer of
liquid water, perhaps as much as 50 km deep, kept liquid by
tidally generated heat.
If so, it would be the only place in the solar system besides
Earth where liquid water exists in significant quantities.

Europa's most striking aspect is a series of dark streaks
crisscrossing the entire globe.
The larger ones are roughly 20 km across with diffuse outer edges and a
central band of lighter material.
The latest theory of their origin is that they are produced by a series of
volcanic eruptions or geysers.

Recent observations with HST reveal that
Europa has a very tenuous atmosphere (1e-11 bar) composed of oxygen. Of the 61
moons in the solar system only four others (Io,
Ganymede,
Titan and Triton)
are known to have
atmospheres. Unlike the oxygen in Earth's atmosphere,
Europa's is almost certainly not of biologic origin.
It is most likely generated by sunlight and
charged particles hitting Europa's icy surface producing water vapor which
is subsequently split into hydrogen and oxygen. The hydrogen escapes leaving
the oxygen.

The Voyagers didn't get a very good look at Europa.
But it is a principal focus
of the Galileo mission.
Images
from Galileo's first two close encounters with Europa
seem to confirm earlier theories that Europa's surface is very young:
very few craters are seen, some sort of activity is obviously occurring.
There are
regions
that look very much like
pack-ice on polar seas during spring thaws on Earth.
The exact nature of Europa's surface and interior is not yet clear but the evidence
is now strong for a subsurface 'ocean'.

Galileo has found some evidence of a weak magnetic field (perhaps 1/4 of the strength of Ganymede's).